Ruger rimfire pistols are fixed barrel guns and the serial number is on the receiver, not the frame. So if you found someone selling just the upper receiver, less the bolt, it would still have to go thru an FFL in CA. And most likely they wouldn't do it anyway because the DOJ requires transfer of a 'complete' gun for pistols. The MK II/III and 22/45's are not that expensive, just buy the complete gun. There is like 70 plus on gunbroker.com

A guy on the rimfire forum in Texas has a few 10" bull barrel ruger uppers to sell. I figured I could make myself a long barrel ruger if I could get 1 of those uppers, swap in my mk iii bolt and lower.